I. Field of the Invention
The current invention relates to wireless communications. More particularly, the present invention relates to a novel apparatus for the use of beam sweeping techniques to provide greater capacity in a multi-user wireless communication system.
II. Description of the Related Art
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system which conforms to the xe2x80x9cTIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, hereinafter referred to as IS-95. The CDMA system allows for wireless voice and data communications between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein.
The International Telecommunications Union recently requested the submission of proposed methods for providing high rate data and high-quality speech services over wireless communication channels. A first of these proposals was issued by the Telecommunications Industry Association, entitled xe2x80x9cThe cdma2000 ITU-R RTT Candidate Submission, xe2x80x9d hereafter referred to as cdma2000 and incorporated by reference herein. Methods of transmitting user data (non-voice data) over fundamental and supplemental channels are disclosed in cdma2000.
In a CDMA system, a user communicates with the network through one or more base stations. For example, a user on a subscriber station communicates to a land-based data network by transmitting data on the reverse link to a base station. The base station receives the data and can route the data through a base station controller (BSC) to the land-based data network. The forward link refers to transmission from the base station to a subscriber station and the reverse link refers to transmission from the subscriber station to a base station. In IS-95 systems, the forward link and the reverse link are allocated separate frequencies.
The subscriber station communicates with at least one base station during a communication. CDMA subscriber stations are capable of communicating with multiple base stations simultaneously during soft handoff. Soft handoff is the process of establishing a link with a new base station before breaking the link with the previous base station. Soft handoff minimizes the probability of dropped calls. The method and system for providing a communication with a subscriber station through more than one base station during the soft handoff process are disclosed in U.S. Pat. No. 5,267,261, entitled xe2x80x9cMOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein. Softer handoff is the process whereby the communication occurs over multiple sectors which are serviced by the same base station. The process of softer handoff is described in detail in copending U.S. Pat. No. 5,625,876, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION, xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein.
Given the growing demand for wireless data applications, the need for very efficient wireless data communication systems has become increasingly significant. The IS-95 standard is capable of transmitting traffic data and voice data over the forward and reverse links. A method for transmitting traffic data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled xe2x80x9cMETHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION, xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein. In accordance with the IS-95 standard, the traffic data or voice data is partitioned into code channel frames of 20 milliseconds in duration with data rates as high as 14.4 Kbps (kilo-bits-per-second).
A significant difference between voice services and data services is the fact that the former imposes stringent and fixed delay requirements. Typically, the overall one-way delay of speech frames must be less than 100 msec. In contrast, the data delay can become a variable parameter used to optimize the efficiency of the data communication system. Specifically, more efficient error correcting coding techniques which require significantly larger delays than those that can be tolerated by voice services can be utilized.
The parameters which measure the quality and effectiveness of a data communication system are the transmission delay required to transfer a data packet and the average throughput rate of the system. Transmission delay does not have the same impact in data communication as it does for voice communication, but it is an important metric for measuring the quality of the data communication system. The average throughput rate is a measure of the efficiency of the data transmission capability of the communication system. An exemplary system for providing data services over a wireless link is described in copending U.S. patent application Ser. No. 08/963,386, entitled xe2x80x9cMETHOD AND APPARATUS FOR HIGHER RATE PACKET DATA TRANSMISSION, xe2x80x9d now U.S. Pat. No. 6,574,211, issued on Jun. 3, 2003, which is assigned to the assignee of the present invention and included herein by reference.
In a CDMA communication system, capacity is maximized when the transmission energy of signals is kept to the minimum value that will satisfy reliability performance requirements. The reliability in reception of a signal depends on the carrier-to-interference ratio (C/I) at the receiver. Thus, it is desirable to provide a transmission power control system that maintains a constant C/I at a receiver. Such a system is described in detail in U.S. Pat. No. 5,056,109 (the ""109 patent) entitled xe2x80x9cMethod and Apparatus for Controlling Transmission Power in a CDMA Cellular Telephone System, xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein.
In the ""109 patent, a closed loop power control system is described in which the C/I (referred to in the ""109 patent as signal to noise ratio) is measured at the receiver and compared to a single threshold value. When the measured C/I exceeds the threshold, a power control command is sent requesting the transmitter to decrease its transmit power. Conversely, when the measured C/I falls lower than the threshold, a power control command is sent requesting the transmitter to increase its transmit power. Because the C/I is not the only factor that determines the reliability of reception of a signal, the ""109 patent also describes an outer loop power control system that varies the threshold value in order to satisfy a target reliability.
It is well known that in cellular systems the carrier-to-interference ratio (C/I) of any given user is a function of the location of the user within the coverage area. In order to maintain a given level of service, TDMA and FDMA systems resort to frequency reuse techniques, i.e. not all frequency channels and/or time slots are used in each base station. In a CDMA system, the same frequency allocation is reused in every cell of the system, thereby improving the overall efficiency. The C/I that any given user""s subscriber station achieves determines the information rate that can be supported for this particular link from the base station to the user""s subscriber station.
It is also well known that much of the signal interference in a loaded CDMA system is caused by the loaded system""s own transmissions, from both base stations and subscriber stations. Transmissions from a base station to subscriber stations, also referred to as forward link transmissions, cause interference within its own and neighboring cells. Transmissions from subscriber stations to base stations, also referred to as reverse link transmissions, cause interference to the reverse link transmissions of other subscriber stations. In order to maintain a C/I that allows reliable communications, base stations and subscriber stations in a loaded CDMA system transmit more power to overcome the interference. The capacity of the wireless communication channel limits the sum of the power transmitted on the forward and reverse links. Thus, the more power transmitted on the forward or reverse links for each subscriber station, the fewer subscriber stations the system can support. Therefore, there is a need to identify methods of decreasing the forward and reverse link power without sacrificing C/I.
The present invention provides an improved-capacity wireless system by employing beam steering techniques to decrease the required transmit power of base stations and subscriber stations in the system. A base station in the improved wireless system decreases the interference it causes to neighboring cells by transmitting forward link signals along narrow signal beams. Subscriber stations located in a neighboring cell or sector.
To improve the carrier-to-interference ration (C/I) on the forward and reverse links, base station designs have been proposed that utilize narrow, moving signal beams instead of or in addition to broad beam coverage of sectors. For example, such a base station might transmit signals to subscriber stations within its sectors using narrow signal beams that cover a fraction of each sector at any point in time.
Reverse link signals traveling within a signal beam suffer little interference from signals originating from outside the signal beam, because the latter is greatly attenuated by the directional antenna. Consequently, such reverse link signals may be transmitted at lower power than would be necessary with a broad signal beam, thus reducing interference to neighboring coverage areas.
On the forward link, transmitting through a narrow beam reduces the interference that a transmitting base station causes to neighboring coverage areas. When multiple base stations of a wireless communication system transmit through narrow signal beams, they reduce the average interference caused by each base station to its neighbors. This reduction in interference allows forward link signals to be transmitted reliably at lower power.
As described above, enabling reliable communication with less transmission power allows increased capacity in a wireless communication system. Therefore, a method and apparatus that facilitates wireless communication through narrow beams is highly desirable. Additionally, methods of providing signal beams that sweep reliably and steadily through coverage areas are highly desirable. When a base station is divided into a conventional three-sectored configuration, it is desirable to have at least one signal beam sweeping through each sector at any given moment in time. Relatively uniform coverage is provided if such a signal beam always sweeps through the sector in the same direction.
In order for a signal beam to sweep in the same direction through a 120-degree sector area, the signal beam needs to be able to jump from one sector edge to the other after each sweep. In other words, the signal beam must sweep from one edge, through the sector coverage area, to the opposite edge and immediately begin sweeping again at the first edge. Such signal beam movement is possible with non-mechanical beam steering mechanism such as using a phased antenna array, but such non-mechanical methods are expensive and complex to implement. It is less expensive to use mechanical means to steer the signal beam, such as a rotating directional dish antenna. The problem with a physically rotating dish is that it is very difficult to make large, sudden beam angle changes, such as changing quickly from one edge of a 120-degree sector to the other.
The preferred embodiment of the present invention provides three signal beams that sweep steadily through each sector of a conventional three-sectored cell. The beams sweep in one direction, thus providing relatively uniform beam coverage of all parts of each sector. In the preferred embodiment, the three signal beams emanate from each of three directional dish antennas, mounted onto an antenna assembly such that each antenna is directed at approximately 120-degree angles from the others. When this antenna assembly is rotated in one direction at the center of a conventional three-sectored cell, the resulting three signal beams cross sector boundaries at the same time.
In the preferred embodiment, the base station equipment associated with forward and reverse link communication for each sector is connected to a signal switch. The signal switch routes forward and reverse link signals such of each sector through one of the three dish antennas at any instant in time. When the three signal beams sweep to the boundaries between sectors, the signal switch changes the routing of signals to and from the base station equipment for each sector to the dish antenna whose beam is just entering the sector. In other words, as each signal beam crosses from one sector into another sector, the signals for that beam are switched from the sector it is exiting to the sector it is entering. From the perspective of the base station equipment for a single sector, the sector""s signal beam sweeps uniformly from one edge of the sector to the other, and immediately changes positions to the first edge of the sector again.
The sweeping of a signal beam is referred to herein as beam sweeping, and a base station employing beam sweeping techniques is referred to herein as a beam sweeping base station.